An open fire has been used as a heating source in many homes since ancient times. However, in an open fire the fuel in form of firewood is consumed quickly and reproduces only a fraction of the energy in the form of radiant heat. Most of the energy disappears with the flue gases out through the chimney. An open fire also consumes a lot of the air in the room and this air must be replaced, which occurs with cold air from outside that penetrates through slots in doors and windows. This creates floor draft and coldness in the room.
Today, we often use a stove instead of an open fire as a secondary heating source in our homes. In stoves, the combustion of fuel typically takes place in a closed combustion chamber, which is provided with air from outside through the wall or bottom plate. In order to have an effective combustion process, it is important to regulate the amount of air supplied to the combustion chamber. One way of regulating the air supplied to the combustion chamber is to have the stove door partly open during the ignition of the fire and then to close the stove door when the fire has been established. However, modern houses are often very tight and designed with well defined active air removal system stipulating a specific air flow out of the house. In such modern house there is a slight under-pressure in the house. This under-pressure may also be caused by a strong kitchen fan. In such a house, it would not be acceptable to have the stove door partly open; it would counteract the active air removal system and the under-pressure in the house would draw smoke from the fire into the indoor environment instead of the smoke being removed via the chimney. Moreover, the habit of having the stove door partly open introduces safety hazards; a glow may jump out through the partly open stove door and cause a fire or the door may accidentally be opened fully introducing the risk of burning yourself on the fire.
To address this issue there exist a number of more or less complex designs where the air supply is regulated by different kinds of valves or throttles. The regulation of the combustion air is often made by one or more valves manually operated by one or more levers placed on the front of the stove.
DE202007003345U1 discloses an example of a manually operated valve. The air is supplied in a primary channel from below through a riddling gate and a secondary channel from above along the front of the combustion chamber. The supply of air in the channels is regulated by a damper operated via a knob on the front side of the stove so that the supply of air in the primary channel may be completely stopped and the supply of air in the secondary channel is throttled upon reaching a sufficient combustion temperature.
However, it is difficult for the user to perform this regulation correctly to achieve an efficient burning of the fire wood through the duration of the burning of the firewood. It is also common that the user is not aware of the need to adjust the air flow or that the user forgets to adjust the air flow after ignition of the fire. In any case it will result in a too rapid combustion of the firewood.
To address this issue there exist a number of more or less complex designs where the air supply is automatically regulated by different kinds of automatically controlled valves or throttles.
DE202015101389 U1 discloses a fireplace cassette for indoor use, where the combustion chamber is provided with two separate air flows from the outside, a primary channel for supplying air from below and a secondary channel for supplying air from above along with the front side of the fireplace cassette. These air supply channels are regulated by separate dampers controlled by a gel, whose volume increases with increased temperature in the combustion chamber so that the gel is pressing a spring, which in turn acts on a damper arm for adjusting the position of the dampers.
A similar system is disclosed in U.S. Pat. No. 4,265,213 in which a primary channel for supply of air from below and a secondary channel for supply of air from above along the front of the combustion chamber. Both channels have a common inlet but different dimensions for regulation of the air flow. The common inlet is regulated by a damper, having a plate mounted for rotation with a horizontal shaft centrally driven by a bimetallic temperature-activated coil.
DE10012485 A1 discloses an air supply controller for a stove. The controller consists of a bimetallic sensor that controls a valve dependent on the temperature in the combustion chamber for regulation of the supplying of outside air. This document discloses the use of three bimetallic controlled valves wherein all three valves are controlled to open for a greater air flow when the fire burns hot and wherein all three valves are controlled to close for a smaller air flow when the fire burns at a lower temperature.
WO 2008/046425 A2 relates to a method for controlling the supply of combustion air to a combustion chamber, where the air supply is controlled in accordance with a predetermined program as a function of the combustion time.
U.S. Pat. No. 6,216,684 B1 discloses a combustion system for burning firewood including a combustion chamber defined by front, rear and side walls, a ceiling and a bottom. An access door is provided for addition of fuel into the combustion chamber. A substantial amount of combustion air enters the combustion chamber near the top of the fuelling doors via apertures and is directed down the face of the fuelling doors providing cooling. This document discloses that when the fire is during start-up more air should be provided from above the combustion chamber. This document also discloses the use of bi-metallic coil or strip that controls the air flow such that less air is supplied from above the combustion chamber when the fire burns hotter.
These automatically control methods are complex, difficult and expensive to manufacture. It is also an object to control the air supply for obtaining a clean and complete combustion with maximum heat recovery. Moreover, the above automatically controlled systems are difficult to combine with manual operation, which e.g. may be desired when adding firewood to an almost burnt out fire.